| The diverse geology and geotectonic setting of North Pakistan is favorable for a wide range and variety of minerals deposits,such as arc and back-arc epithermal precious metal deposits,arc-related Cu-(Mo)-(Au)-porphyries,Kuroko-type polymetallic massive sulphides,polymetallic skarns and veins,SEDEX Pb-Zn deposits;Ni-Co-Cu sulphide,orogenic gold deposits and PGE.However,due to rugged and glaciated terrain of the western Himalaya,the exploration activities have been limited to stream sediments and no hydrothermal economically feasible gold deposit was discovered till date and the mineralsation type,ore genesis,crystallization temperatures,and source rocks of gold deposits contributing placer gold in the downstream valleys are still unknown.In this study,three anomalous localities(Bagrote,Dainter and Shimshal)in terms of placer gold were selected for case studies to investigate the above mention research issues form the Kohistan island arc and the Karakoram Block of the north Pakistan.The techniques of Scanning Electron Microscopy(SEM),Electron Probe Micro Analysis(EPMA)and Electron Dispersive Spectroscopy(EDS)were used to study the detail morphology,and microchemistry of gold grains,while a multiparametric approach on pyrite including XRD analysis,thermoelectricity,major and trace element geochemistry,EPMA X-ray elemental maps and isotopic characteristics,was employed to constrain the nature of deposits,mechanism of gold enrichment in pyrite,source of ore forming fluids and metals,source lithology and denudation or erosion rate of the deposits in the hinterlands contributing placer gold in the downstream valleys.The detrital zircon U-Pb geochronology and composition was studied by laser ablation inductive couple mass spectroscopy(LA-ICPMS)to constrained the age of the gold deposits and trace element fertility indicators of the detrital zircon were used to discriminate prospective(fertile)lithologies for mineral deposits and unprospective(infertile)suits on the hinterland of the studied areas(Bagrote,Shimshal and Dainter valleys).The placer gold grains display varying morphologies ranging from irregular to sub-rounded and rounded.Majority of grains from the Bagrote and Dainter valleys exhibit crystal outlines still preserved,indicating short distance of transport,while some grains are lack of crystal outline showing sub-rounded to rounded outlines indicating long distance of transport.Majority of placer gold grains from the Shimshal valley display sub rounded to rounded morphologies indicating comparatively long distance of transport.Some gold grains from the Bagrote and Dainter valleys display hammering,groves and cavities on the surface which reveal the grains have travelled less than 1000 m because the hammering is rarely seen in the grain traveled greater than 1000 m.Irregular and rounded morphologies in same sample location shows multiple deposits on varying distances in the Bagrote,Shimshal and Dainter valleys.Notably,majority of gold grains from the three studied locations from upstream and downstream do not contain jagged outlines,this is because majority of the grains were subjected to glacial transport before transferring to fluvial system.The gold grains display a microtexture typical of mechanical abrasion rather than chemical activity.Their chemical composition of most of the grains from each stream falls in a restricted range mainly minor alloying elements(Cu,Hg)with a restricted range of fineness(1,000 Au/(Au + Ag)in each locality,suggesting derivation from a homogeneous source or limited number of mineralization types.Gold grains from the three localities show homogenous chemical composition from core to rim and,are alloys of Au and Ag as the qualitative EDS analysis show no peaks for elements other than Au and Ag.Ag is highest in the gold grains from the Shimshal locality(18.5%)followed by Dainter(6.2%)and Bagrote(mean 1.9%)localities.while Cu is highest in the gold grains from Bagrote locality(0.39%),followed by the Dainter and Shimshal localities which show Cu below detection limit in most of the grains.The chemical composition of placer gold and auriferous pyrite in triangular diagrams in terms of Au–Ag–Cu and As-CoNi respectively,and on the basis of comparison of the trace elements content,?Fe/?S-As and(Fe+S)-As characteristics of the auriferous pyrite with different genetic types of gold deposits reveal that the placer gold and the detrital auriferous pyrite were derived from porphyry epithermal type of magmatic hydrothermal deposits.The EPMA-X ray elemental maps of auriferous pyrite grains from the Bagrote,Shimshal and Dainter valleys show generally homogeneous distribution from core to rim which indicate stable physiochemical conditions during crystallization.The pyrite grains display euhedral to subhedral morphologies and euhedral pyrite,mostly cubic,occurs as secondary grains while the rounded morphologies are rare.Crystal habits from the Bagrote locality include {-1 1 1},{1 0 0},{2 0 0},{-1 0 2},{-2 1 1},{0-2 2},{-1-1 3},{2-2 2},{0-2 3} and {-1-2 3}.While the crystal habits from the Shimshal locality mainly contain {-1 1 1},{1 0 0},{2 0 0},{-1 0 2},{-1-1 2},{0-2 2},{-2 1 1} {-1-1 3},{2-2 2},{0-2 3},and {-1-2 3};from Dainter locality chief crystal habits are {-1 1 1},{1 0 0},{-2 0 0},{-1 0 2},{-1-1 2},{0-2 2},{-1-1 3},{2-2 2},{0-2 3} and {-1-2 3}.The measured thermoelectric coefficients of the pyrite of the placer deposits of the Bagrote valley range from-317 ?V/? to 303 ?V/? and the occurrence rate of P-type is 10.9%,while N-type occurrence rate is 89.1% and concentrated at-150 to-50 ?V/?.The measured thermoelectric coefficients of the pyrite of the placer deposits of the Shimshal valley range from-303.2 ?V/? to 305.3 ?V/? and the occurrence rate of P-type is 41.8%,concentrated at 50 ?V/? to 300 ?V/? while N-type occurrence rate is 58.2% concentrated at-150 to-50 ?V/? While the thermoelectric co-efficient of pyrite from the Dainter valley placer deposits ranges from-395.5 ?V/? to 334.5 ?V/? and the occurrence rate of p-type is 33.2% while N-type pyrite is 66.8% forming strong peaks as compared to P-type.The temperatures calculated using thermoelectric coefficient estimate from the Bagrote,(290°C to 380°C),Shimshal(315 °C to 375 °C followed by 210°C to 270°C and 115°C to 150°C)and Dainter localities(280°C to 360 °C)are also consistent with porphyry epithermal type of mineralisation.The values of Xn P of pyrites in the Bagrote,Shimshal and Dainter valleys placer deposits vary from-105 to-82 with a mean of-90.7,-69 to-14 having a mean of-48.7 and-90 to-25 with an average of-60.5 respectively.While the values of ?(exhumation level= ? = 50 – Xn P/4)of the pyrite from the Bagrote,Shimshal and Dainter valleys have a mean value 71.8%,62.8%,and 65.3% respectively.The Xn P and corresponding ? values of pyrite from the Bagrote and Dainter valleys suggest that the deposits on the hinterlands have been significantly eroded,while the deposits on the catchment of the Shimshal valley have not been significantly eroded.These values also suggest the orebody on the catchment of the Bagrote and Dainter valleys likely represent the lower to middle part of the deposit,while in the Shimshal valley the value of ? reveals that the detrital pyrite was derived from the upper parts of the ore body.The calculated chemical formula of auriferous pyrite from the Bagrote [(Au0.0006Fe)S2.004],Shimshal [(Au0.0003Fe)S2.004] and Dainter valleys [(Au0.0003Fe)S2.013] placer deposits combined with Au-As and Fe-Au plots show that Au is enriched in pyrite as micro to nano inclusion as Au0,instead of Au+1.The generally high Mo/Ni ratios of pyrite from the Bagrote,Shimshal and Dainter localities reflect highly differentiated magmatic rocks i.e.the Chalt Volcanics and Kohistan batholith on the catchment of the Bagrote valley while Hunza plutonic unit in the catchment of the Dainter and Shimshal valleys.The ?34 SV-CDT values from the Bagrote and Shimshal valleys range from – 0.6 to 0.9 with a mean of-0.02 and from-5.3 to-3.8 with mean of-4.7 respectively indicating a magmatic homogeneous sulfur source.The values of 206Pb/204 Pb,207Pb/204 Pb and 208Pb/204 Pb for pyrite from placer deposits of the Bagrote locality show narrow range from 18.3775 to 18.4337 with an average of 18.395,from 15.5803 to 15.6032 with an average of 15.587,and from 38.4451 to 38.5469 with a mean of 38.476 respectively.While from the Shimshal locality Pb isotope values range from 18.487 to 18.692(206Pb/204Pb),15.713 to 15.745(207Pb/204Pb)and 38.964 to 39.416(208Pb/204Pb).These values from the Bagrote locality suggest that Pb,by inference the ore metals originated from orogenic belts with minor contribution of lower crust.The narrow variations in 206Pb/204 Pb suggest a single lead source.While the 206Pb/204 Pb vs.208Pb/204 Pb data from the Shimshal locality propose a mixed Pb source,and might be derived from orogenic belts with considerable contribution of lower and upper crustal sources.The comparatively wide range of Pb isotope values of pyrite from the Shimshal valley also suggest mixed sources.The U-Pb geochronology,texture and composition of the detrital zircon report two age suits(92-78 Ma and 72-62Ma)from the Bagrote locality,fiver populations ages(110-100 Ma,596-507 Ma,879-605 Ma,1196-902 and 2506-1800)from the Dainter valley and 4 pupations of ages(109-95 Ma,124-112 Ma,940-519 Ma and 2746-1696 Ma)from the Shimshal valley placer gold deposits.Among these age suits,the fertile suits were discriminated by elevated Eu/Eu* value >0.3,greater Ce/Nd ratios,higher 10000*(Eu/Eu)/y value >1,Ce/Nd/Y value >0.1 and smaller Dy/Yb ratios.Based on the trace element ratios,REE patterns of different age suits and texture displayed in the CL images,two age suits(92-78 Ma and 72-62Ma)from the Bagrote valley,one age suit(110-100Ma)from the Dainter valley and also one age population(109-95 Ma)from the Shimshal valley is proved to be fertile and hosting porphyry epithermal type of deposits which contribute placer deposits downstream valleys.These fertile age suits from the Bagrote valley(92-78 Ma and 72-62Ma)placer gold deposits are comparable with the Chalt Volcanics and late stage magmatism of the Kohistan Batholith in the Kohistan Island Arc,while the fertile age suits from the Dainter(110-100Ma)and Shimshal valleys(109-95 Ma)are consistent with the ages of post collison granitoids of the Hunza plutonic unit of the Karakoram axial batholith in the Karakoram Block. |
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